Most of the stars in the universe form in young clusters and they are the basic units of star formation (Lada \& Lada 2003). Their study can directly address a number of fundamental astrophysical questions such as the origin and early evolution of stars and planetary systems and more importantly the form and
universality of the stellar initial mass function (IMF). Young clusters in various environments (for e.g., Orion-like high massive star forming regions and Taurus-like low massive star forming regions) provide the ideal laboratories to answer one of the key questions in astrophysics 'What kind of star forming environment would best represent the birth site of our solar system?' The study of IMF and circumstellar disk evolution in young clusters formed in spontaneous and triggered/sequential modes of star formation is important to understand whether or not the IMF and disk properties are indeed universal or not. Hence, one of the major goals of this thesis is to understand the dominant star formation mechanism associated with a given star forming region and hence its influence on its neighborhood in the form IMF and disk evolution.
In the present work, a multi-wavelength observational attempt has been made to study three young clusters (Stock 8, NGC 1624 and NGC 2175), situated at >2 kpc and associated with HII regions and are suggested to have diverse star forming environments. Chapter 1 deals with the introduction to the general characteristics of young embedded clusters, star formation, evolutionary phases of pre-main-sequence (PMS) stars, modes of triggered star formation around young clusters, IMF and the selection of objects for the present study. In this observational project, we have carried out multi-wavelength studies from optical to radio bands to make a comprehensive exploration of the issues mentioned above. Chapter 2 deals with the details of various observations, data reductions and calibration procedures for the various data sets.
In Chapter 3 we have studied the young cluster Stock 8 associated with the HII
region Sh2-234 using deep optical UBVI and narrow-band data using SII, Hα,
slitless spectroscopy along with archival data from the surveys such as 2MASS, MSX, IRAS and NVSS. The radial extent of the cluster and Reddening (E(B-V)) in the direction of the cluster are found to be 6' and 0.40-0.60 mag, respectively. The post-main-sequence age and distance of the cluster are found to be < 2 Myr and 2.05±0.10 kpc, respectively. Using NIR excess colour criteria from (J-H)/(H-K) CC diagram and grism survey for Hα emission stars, candidate YSOs within the region have been identified. A significant number of YSOs are found to be aligned along a Nebulous Stream eastwards of the cluster and an embedded cluster is located along the Stream at a distance of ~13' from the center of Stock 8. The mass of the candidate YSOs lie in the range of ~0.1–3.0M☉.and have ages ~1-5 Myr, suggests that star formation in the cluster may be non-coeval. The embedded cluster in the Nebulous Stream is found to be physically connected to the HII region. The YSOs located in the Nebulous Stream, embedded cluster and in the western region of Stock 8 have larger (H-K) excess in comparison to those located in the central region of Stock 8. The (H-K) excess, V/(V-I) and K/(H-K) CMDs indicate that these YSOs are younger than those located in the central region of Stock 8.
The radio continuum, MSX, IRAS maps and the ratio of SII/Hα intensities indicate that the eastern region of the cluster is ionization bounded, whereas, the western region of the Stock 8 is density bounded. The HII region contains nearly 13 OB stars (Meyer \& Macak 1971), most of which are located towards west of Stock 8. Presumably these massive stars are the first generation stars in the region and Stock 8 would have formed under the influence of these stars. The morphology of radio emission and MSX A-band emission suggests that ionization/shock front caused by the central ionization source of Stock 8 and by the O type stars located in the western region of Stock 8, has not reached the Nebulous Stream. An O8 star located ~9 pc away towards the north of the Nebulous Stream may be a probable source of the bright-rimmed Nebulous Stream. The slope of the mass function Γ, in the mass range ~1.0< M/ M☉< 13.4 can be represented by -1.38 ±0.12, which agrees well with Salpeter value (-1.35). In the mass range 0.3< M/M☉<1.0 the mass function is found to be shallower with Γ=-0.58±0.23 indicating a break in the slope of the MF at ~1M☉. The slope of the KLF for the cluster is found to be 0.31±0.02 which is fairly in agreement with the value of KLF obtained for other young clusters.
In Chapter 4 we have studied the young cluster NGC 1624 associated with the
HII region Sh2-212 using multi-wavelength data sets such as optical UBVRI photometry, optical spectroscopy, radio continuum observations along with 2MASS, MSX and IRAS data. The radial extent and E(B-V) of NGC 1624 are found to be 2' and 0.76-1.00 mag, respectively and distance is estimated to be 6.0±0.8 kpc. The maximum post-main-sequence age of the cluster is estimated as ~4 Myr. Spectroscopic analysis of the massive ionizing source indicates a spectral class of O6.5V. Using JHK colour criteria, 120 candidate YSOs are identified and majority of the YSOs have Av < 4.0 mag and masses in the range ~0.1-3.0M☉. Distribution of these YSOs on the CMD indicates an age spread of ~0.5-5 Myr with an average age of ~ 2-3 Myr, suggesting non-coeval star formation in NGC 1624. The NIR excess fraction on the basis of JHK data is found to be ~20% which indicates an average age ~2-3 Myr for YSOs. From the radio continuum flux, spectral type of the ionizing source of the UCHII region is estimated to be ~B0.5V. A significant number of YSOs are located close to the cluster center and a few YSOs are seen to be located/projected over the molecular clumps detected by Deharveng et al. (2008), as well as farther away from the clumps. An enhanced density of reddened YSOs located/projected close to the molecular clump C2 is detected and are found to be younger than those located within the cluster core. The slope of the MF, Γ, derived from optical data, in the mass range 1.2☉<27 can be represented by -1.18±0.10, whereas NIR data yields Γ= -1.31±0.15. Slope of the KLF in the magnitude range 13.5 - 17.5 is found to be 0.30±0.06 which is in agreement with the values obtained for Stock 8 and other young clusters of similar ages.
In Chapter 5 we have studied the young cluster NGC 2175 which is a part of the HII region Sh2-252. The central massive source HD 42088 is thought to be the ionizing source of this region. Felli et al. (1977) detected 6 radio sources within Sh2-252 of which four of them (Sh2-252 A, B, C and E) are classified as compact HII regions. An optically bright cluster (NGC 2175s) is located towards the east of Sh2-252. We have carried out an extensive multi-wavelength analysis of the region using the UBVRI data, slit and slitless spectroscopic data, JHK data from 2MASS, MIR data from Spitzer-IRAC and MIPS. We have carried out optical spectroscopic analysis of 15 bright sources of the region of which eight have been identified as massive members of spectral class earlier than B3. From the spectroscopic analysis, the distance of the region has been estimated as 2.4±0.2 kpc and E(B-V) of the massive members varies between 0.35-0.75 mag. Four of the bright sources in NGC 2175s are found to be earlier than B2 and its distance seems to be matching with that of Sh2-252 which suggests that NGC 2175s is a part of Sh2-252. The optical CMD and 2MASS CC diagram analysis show that the sources with (H-K) >0.35 mag could be the probable PMS members of the cluster and they are distributed within the age range of 1-10 Myr. The IMF of NGC2175s has been obtained as Γ=-0.71±0.06. The YSOs of the Sh2-252 region has been identified and classified usng the Spitzer-IRAC data along with the 2MASS NIR data. 131 Class I and 400 Class II sources and 61 Hα emission line sources have been identified. Majority of the YSOs are found to have an age distribution between 0.1-5 Myr and mass range of 0.3 - 2.5M☉.The KLFs of the sub-regions Sh2-252 A, C, E and NGC 2175s have been obtained as 0.23±0.06, 0.29±0.03, 0.19±0.03, 0.18±0.02, respectively. Within errors, the KLFs are similar for all the sub-regions suggesting that they might be having similar age distribution. The spatial distribution of YSOs shows that most of them are crowded near Sh2-252 A, C and E. We have conducted the SED analysis of 142 YSOs in the optical to 24 μm wavelength range. The Class II YSOs are found to have the disk accretion rate of the order of ~10-7-10-8 M☉ yr-1 and the sub-regions Sh2-252 A, C and E are having similar age range of ~2-3 Myr, suggesting that they are formed at the same time. Five dark filamentary structures are identified across the region and the YSOs are seen to be nicely aligned along these sructures. The correlation between the 850 μm emission and the dark features identified at 40” east of Sh2-252 A suggests that these filamentary structures are possibly a part of an IRDC, in which the star formation has been started already. The SED based age analysis shows that the YSOs on the filamentary structures are having an average age of ~0.5-1 Myr, suggesting that they are formed at a later evolutionary stage of Sh2-252. The age sequence between the ionizing source and the recently formed YSOs on the filamentary structures and the morphology suggests that they are the probable second generation stars in the region formed due to the strong interaction of the radiation from the expanding HII region to its surroundings.
Chapter 6 concludes the research work on the thesis based on the analysis of three young clusters.27>